An image forming apparatus (e.g., copier, laser beam printer) may employ an optical scanning system having an optical element such as a f-theta lens, and a projection lens.
With an increased demand on cost reduction on a finished product, materials used for such optical element have been shifted from glass material to resinous material.
For a long period of time, such optical element has been made as an optical element having a spherical surface shape such as a convex-shaped lens and a concave-shaped lens.
However, such optical element having a spherical surface shape may need a plurality of optical elements (e.g., two to three elements) to correct an optical error (e.g., picture out of focus) because such optical error may not be corrected by one optical element.
If the number of optical elements used in an optical scanning system increases, it may unfavorably increase the manufacturing cost, and may unfavorably restrict the freedom of structural layout of the optical scanning system.
Accordingly, it is preferable to use a smaller number of optical elements in an optical scanning system to reduce the manufacturing cost and to increase the freedom of structural layout while maintaining or improving an optical function of the optical scanning system.
Such optical scanning system having a smaller number of optical elements may be realized by using an optical element having a non-spherical surface shape, which may have a complex pattern.
With such optical element having a non-spherical surface shape, an optical scanning system may conduct multiple functions with a smaller number of optical elements, which may be preferable from a viewpoint of reducing the manufacturing cost and increasing the freedom of structural layout.
For example, such optical element having a non-spherical surface shape may include a lens having an uneven thickness.
Such lens having an uneven thickness may be made by an injection molding method, which is commonly used for mass producing at a lower manufacturing cost.
However, such lens (e.g., having an uneven thickness) manufactured by an injection molding method may have drawbacks as set forth below.
A lens having an uneven thickness may inevitably have deviation (or unevenness) of lens thickness in different portions of the lens. When manufacturing such lens by an injection molding method, a resin material filling the mold of an injection molding machine can have different shrinking rates at different portions of the filled resin material, such that a lens having an uneven thickness is not formed with high precision.
For example, such filled resin material can have thermal stress at its thin-walled portion or at a portion near a gate of the injection molding machine. Accordingly, a lens having unfavorable quality such as double refraction (or birefringence), for example, may be manufactured.
Several methods have been devised in the past for reducing such drawbacks of injection molding method. For example, one such method is conducted as below.
A resin material is heated in a metal mold to its glass transition temperature or higher, and the resin material is then slowly cooled. After such cooling, the molded object made of resin material may be removed from the metal mold. Such method suppresses the occurrence of internal strain in a molded object to obtain a molded object having a higher shape precision.
In another method, a resin material prepared in advance is set in a metal mold, and then such metal mold is heated to the glass transition temperature of the resin material or higher to melt the resin material. The resin material is slowly cooled thereafter. In such a process, an internal pressure can build up in the resin material.
In yet another method, the metal mold may be heated to a glass transition temperature of a resin material or higher, and then the resin is injected and fills the heated metal mold for a given period of time. Thereafter, the metal mold is cooled to a temperature lower than the glass transition temperature of the resin material, and then the molded object made of resin material may be removed from the metal mold.
In yet another method, a resin material fills the cavity of the metal mold and is heated to its glass transition temperature or higher. The resin material is then cooled to a temperature near the glass transition temperature while maintaining the pressure applied to the resin material at substantially the same level during the cooling process by adjusting the pressure applied from an external side of the metal mold.
The above-mentioned methods require heating a metal mold to a temperature greater than the glass transition temperature of the resin material, and then cooling the metal mold.
Accordingly, the molding cycle is significantly longer. Furthermore, because the metal mold must to be heated to a glass transition temperature of the resin material or higher for each molding cycle, such methods require greater power consumption.
In another related method, an injection molding method is conducted as described below, which is explained with reference to FIGS. 1A and 1B.
As shown in FIG. 1A, injection molding may be conducted, for example, with metal molds 101 and 102 having a transfer face portion 103 and a non-transfer face portion 104, and a movable mold 105.
As shown in FIG. 1A, the mold unit 101 and 102 may be configured to set the non-transfer face portion 104 in a given position with respect to the transfer face portion 103. The non-transfer face portion 104 may not be used for transferring a shape or pattern to a resin, which may become an molded object.
The movable mold 105 is slidable within the mold unit 101 and 102 as shown in FIG. 1B.
A melted resin 106 is injected and fills the cavity formed by the metal molds 101 and 102 and the movable mold 105.
Then, a pressure is applied to the transfer face portion 103 to closely contact the resin 106 to the transfer face portion 103.
Then, the melted resin 106 is cooled to a temperature lower than the softening temperature of resin 106, and a molded object made of resin 106 may be removed from the metal molds 101 and 102.
As shown in FIG. 1B, during a time period for cooling the melted resin 106 to a temperature lower than the softening temperature of the resin 106, the movable mold 105 slides in a direction away from the resin 106 to form a void 107 between the resin 106 and the movable mold 105.
In such molding method, the non-transfer face portion 104 facing the void 107 may not contact a wall face of the metal molds 101 and 102, wherein the resin 106 is more fluid on such a portion.
Accordingly, as shown in FIG. 1B, a surface sink may preferentially occur in the non-transfer face portion 104, wherein the surface sink may have a concave shape, a convex shape, or both concave and convex shape, for example.
With such method, an occurrence of surface sink in the transfer face portion 103 may be prevented or suppressed, such that a molded object having a higher precision shape is produced.
Furthermore, the metal molds 101 and 102 is not heated to a temperature greater than the glass transition temperature of the resin 106, such that the molding cycle is set to a shorter period of time, and thereby the electric power consumption required for injection molding may be set to a smaller level.
However, the above-explained injection molding method shown in FIG. 1 may not effectively induce a surface sink to a face portion of a given type of lens such as a projection lens 110 shown in FIG. 2.
As shown in FIG. 2, the projection lens 110 may have a transfer face portion 111 and a non-transfer face portion 112, wherein the non-transfer face portion 112 may not be used for transferring a shape or a pattern.
As shown in FIG. 2, the transfer face portion 111 of the projection lens 110 may have a relatively greater area, and the non-transfer face portion 112 may have a relatively smaller area for making a void.
Such non-transfer face portion 112 may be too small to effectively induce a surface sink to the non-transfer face portion 112, such that a surface sink may unfavorably occur in the transfer face portion 111.
In recent years, an optical element may have an optical surface formed with a micro-pattern so that a new optical function can be added to the optical element.
For example, an optical element having a new optical function may be a diffraction lens having diffraction effect, manufactured by forming a diffraction pattern on an optical surface of the lens. The pitch of diffraction pattern may be several times the wavelength of light, for example.
When such diffraction pattern is formed on a fine-finished optical plane of the diffraction lens, an aberration property of an optical system may be improved or wavelength selectivity can be provided to an optical system.
If a microstructure having a pitch (e.g., lattice, pillars, or pit) smaller than the wavelength of light is provided on an optical surface of an optical element, such optical element may have a function that is equivalent to an optical element having a thin film with a given refraction index thereon.
Accordingly, by modifying the arrangement or structure of the microstructure, an optical element may be given a function such as an antireflection function.
In general, a reflected light coming from a fine-finished optical plane may cause a ghost or a flare phenomenon, which may result into a degradation of an image quality.
Such drawback may be suppressed by coating an antireflective layer on an optical element with a vacuum deposition method.
However, a molding method that can form the above-mentioned microstructure directly on an optical surface of an optical element may be favorable than a vacuum deposition method for coating a given layer on an optical element after the molding process from a viewpoint of total manufacturing cost.
In general, in an injection molding method for manufacturing a molded object, a molding cycle may be shortened as below.
For example, a metal mold may be heated and maintained at a temperature lower than a softening temperature of resin material. Then a melted resin material may be injected to fill the metal mold, and then the resin material may be quenched and solidified to manufacture a molded object.
However, such injection molding method may not produce a molded object having a higher precision in some cases.
For example, when an optical element having a diffraction pattern or micro-pattern is manufactured by such injection molding method, a melted resin material, filling the metal mold, may contact a surface of the metal mold.
The surface portion of the melted resin may be instantaneously quenched by such contact condition because the metal mold may be maintained at a temperature lower than a softening temperature of resin material. Because of such quenching, a resin material may not be effectively filled in the micro-pattern.
Accordingly, a resultant molded object may have lower optical quality.
In view of such drawback of injection molding method for transferring a micro-pattern to an optical element, the following related arts may be used.
In one related art, the temperature of a resin material may be set in a temperature range greater than the glass transition temperature of the resin material (e.g., plus 10° C. to 150° C. from the glass transition temperature), and a micro-pattern may be transferred to the resin material to manufacture an optical element.
In another related art, a metal mold may be made of a material having a lower heat conductivity (e.g., 20 W/m·K) so that the resin material may be cooled at a relatively slower rate. By using a metal mold having lower heat conductivity, rapid quenching of resin material may be suppressed.
In another related art, after filling a metal mold with a resin material, such metal mold may be heated to a temperature greater than the glass transition temperature of the resin material, and then pressure may be applied to a transfer face portion to form an optical pattern on the resin material.
In another related art, a mold frame may be made of translucent material, in which a given energy (e.g., light beam) may be irradiated to a resin material to heat the resin material to a glass transition temperature or more, and then pressure may be applied to a transfer face portion to form an optical pattern on the resin material.
However, such related arts may also need a relatively longer molding cycle or time because a relatively longer thermal cycle may be required, wherein the thermal cycle may include a heating process of a metal mold to a glass transition temperature and a subsequent cooling process, for example.
Furthermore, such related arts may not have stable transferability because of temperature instability during the molding process.
Even if a metal mold may be made of a material having a lower heat conductivity as above mentioned, a resin material may be cooled when the resin material flows in the metal mold because the temperature of the metal mold may be maintained at a temperature lower than a softening temperature of the resin material.
Accordingly, such resin material may be further cooled to a lower temperature as the resin material further goes into a portion of a cavity, which is far from the gate of the metal mold. Under such condition, the resin material may not be effectively filled in the metal mold.
Therefore, the resultant molded object may not have a higher quality pattern shape (e.g., micron order shape) thereon.
Furthermore, if a metal mold is made of a material having a lower heat conductivity, the molding time may become longer because the cooling rate of the resin material may become slower. Such molding time may become longer as the thickness of the molded object becomes greater.
Although, a translucent material (e.g., sapphire, silicon) may be used for the mold frame as above-mentioned, such translucent material may be significantly expensive compared to a metal, and may have a lower strength compared to a metal.
Accordingly, the pressure that can be applied to the translucent material may become a smaller value, and thereby a transfer pattern may not be effectively transferred to the resin material.
Furthermore, such translucent material may not be preferable for uniform heating of a molded object having a greater area or curved portion. Accordingly, the resultant molded object may be produced with a lower quality or precision.